tests: posix: pthread: Clean up test for pthread_getname/setname_np

Recently added test for pthread_getname_np()/pthread_setname_np()
functions used uninitialized array value. As we can't check behavior
for arbitrary uninitialized value, but at most NULL value, make the
test calls to explicitly use NULL, to make the test intent clear
(instead of for example initializing the original array with zeros).

Also, these test cases use variable names not consistent with Zephyr
codestyle, and named to generically to cause confuse when used in a
general context of the test_posix_pthread_execution() function, so
rename them to be specific. (And alternative would be to separate
pthread_getname_np()/pthread_setname_np() tests into a separate
function, but it was indeed practical to piggyback them on existing
test_posix_pthread_execution()).

Fixes: #23473

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>